Breath alcohol measurements are known per se and are carried out with different measuring devices and processes. E.g., a measuring device emerges from U.S. Pat. No. 6,167,746 B1 that comprises a graduated tube, to which are connected a pressure sensor and a temperature sensor one after the other, viewed in the direction of gas flow, as well as a gas sampling valve with an electrochemical measuring cell downstream of this valve for the measurement of the breath alcohol concentration.
Prior-art breath alcohol measuring devices, e.g., the Alcotest® devices, have been used for several years for the specific monitoring of the breath alcohol concentration of drivers, especially in traffic checks.
It is well known that breath alcohol measurements can be considerably distorted by the detection of mouth alcohol, since, when mouth alcohol is present, the measured breath alcohol concentration is markedly higher at the beginning of a breathing-out process than at the end. Contrary to this, in the case of a normal respiratory gas sample without mouth alcohol detecting only deep pulmonary gas, the measured breath alcohol concentration increases as a function of exhaled respiratory gas volume or as a function of duration of exhalation. Therefore, a process for detecting the presence of mouth alcohol in a respiratory gas sample has been suggested according to DE 44 43 142 C2, whereby a first respiratory gas sample at the beginning of an exhalation stroke is fed into a measuring cell of a breath alcohol measuring device and a first measurement curve is recorded, and at a second point in time during the same exhalation stroke, if mouth alcohol affects the measurement markedly less, a second respiratory gas sample is fed into the measuring cell of the breath alcohol measuring device and a second measurement curve is recorded. Characteristic parameters, and especially the integral values or maximum values of measurement curves are obtained from each of the two measurement curves and compared to one another, so that, e.g., based on the ratio of the maximum values, it can be determined whether mouth alcohol distorts the measurement, so as to discard the measurement result in this case.
This prior-art process has the drawback that very fast sensors would have to be used for measuring the concentration; thus, depending on the respiratory gas sampling, the measured signal for the first respiratory gas sample has already subsided when the second respiratory gas sample is measured, so that there is practically no longer an overlapping of the two mouth alcohol/respiratory gas alcohol measured effects. Typical practical values for the measurement times are about one second after the beginning of exhalation for the first respiratory gas sampling and about five seconds for the second respiratory gas sampling.
It has now been shown that an exact mathematical separation of the two measured signals cannot be achieved, such that the end-expiratory breath alcohol measurement continues to have errors. These errors are essentially caused by the property of the electrochemical sensors desired in measurement practice that especially measuring sensitivity and reaction speed decrease if these sensors are gassed shortly one after the other. The measurement errors are greater, the higher the alcohol concentration is, e.g., gassing with an alcohol concentration of 1% requires a waiting time of one minute, in order to keep this fatigue effect of the electrochemical sensors negligibly small.